A Review of Terpenes from Marine-Derived Fungi: 2015–2019

Marine-derived fungi are a significant source of pharmacologically active metabolites with interesting structural properties, especially terpenoids with biological and chemical diversity. In the past five years, there has been a tremendous increase in the rate of new terpenoids from marine-derived fungi being discovered. In this updated review, we examine the chemical structures and bioactive properties of new terpenes from marine-derived fungi, and the biodiversity of these fungi from 2015 to 2019. A total of 140 research papers describing 471 new terpenoids of six groups (monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and meroterpenes) from 133 marine fungal strains belonging to 34 genera were included. Among them, sesquiterpenes, meroterpenes, and diterpenes comprise the largest proportions of terpenes, and the fungi genera of Penicillium, Aspergillus, and Trichoderma are the dominant producers of terpenoids. The majority of the marine-derived fungi are isolated from live marine matter: marine animals and aquatic plants (including mangrove plants and algae). Moreover, many terpenoids display various bioactivities, including cytotoxicity, antibacterial activity, lethal toxicity, anti-inflammatory activity, enzyme inhibitor activity, etc. In our opinion, the chemical diversity and biological activities of these novel terpenoids will provide medical and chemical researchers with a plenty variety of promising lead compounds for the development of marine drugs.


Monoterpenes
While monoterpenes were rarely isolated from fungi in the past 15 years, there has been a significant increase in the number of new metabolites reported from marine fungi (only one before 2014 vs. 11 from 2015 to 2019) (1-11, Figure 6) [7,8].

Sesquiterpenes
Sesquiterpenes are the largest group and an excellent source of terpenoids. A total of 45 research papers in 2015-2019 describe 188 new sesquiterpenes  from approximately 19 genera of marine fungi, showing the significant increase in the rate of sesquiterpenes being reported based on the fact that about 208 sesquiterpenes (111 in 2011-2014 and 97 before 2010) were reported from marine fungi by the end of 2014 [7,8].

Sesquiterpenes
Sesquiterpenes are the largest group and an excellent source of terpenoids. A total of 45 research papers in 2015-2019 describe 188 new sesquiterpenes  from approximately 19 genera of marine fungi, showing the significant increase in the rate of sesquiterpenes being reported based on the fact that about 208 sesquiterpenes (111 in 2011-2014 and 97 before 2010) were reported from marine fungi by the end of 2014 [7,8].
An investigation of the extracts from the deep-sea-sediment fungus Aspergillus terreus YPGA10, collected in the Yap Trench at a depth of 4159 m, furnished two new farnesol derivatives,  The production of two new β-bergamotane sesquiterpenoids that are likely important intermediates in the biosynthesis of fumagillin and its derivatives, E-β-trans-5,8,11trihydroxybergamot-9-ene (12) and β-trans-2β,5,15-trihydroxy-bergamot-10-ene (13), were obtained from the marine-derived fungus Aspergillus fumigatus YK-7 isolated from sea mud of the intertidal zone [16]. Compound 12 exhibited weak activities against the U937 cell line with IC50 values of 84.9 μM [16].
An investigation of the extracts from the deep-sea-sediment fungus Aspergillus terreus YPGA10, collected in the Yap Trench at a depth of 4159 m, furnished two new farnesol derivatives, sesquiterpenes aspterric A (14) and aspterric B (15), which are considered to be intermediates in the derivation of aspterric acid [17].
Chemical examination of the deep-sea-sediment-derived Aspergillus sp. SCSIOW2, treated with a combination of 1 mM suberohydroxamic acid (SBHA), a competitive histone deacetylase (HDAC) inhibitor, and 1 mM 5-azacytidine (5-AZA), a DNA methyltransferase (DNMT) inhibitor, led to the isolation of three novel eremophilane-type sesquiterpenes, dihydrobipolaroxin B-D (16)(17)(18). However, 16 and 17 might be artificial products since they could be obtained from the spontaneous intracellular acetalization reaction of dihydrobipolaroxin in water. Moreover, 18 was found to be a mixture of two equilibrium structures in a solution formed through the same acetalization reaction [18]. All of these molecules exerted moderate nitric oxide (NO)-inhibitory activities that were stimulated by lipopolysaccharides (LPSs) and interferon (IFN)-γ in a dose-dependent manner without any cytotoxic effects [18].

Coriolopsis sp.
Two new tremulane sesquiterpenes, coriolopsin A (55) and coriolopsin B (56), were obtained from the EtOAc fraction of the mangrove Ceriops tagal endophytic fungus Coriolopsis sp. J5 [29]. None of these showed obvious cytotoxic or antibacterial activities in bioassays, in which three HTCLs and six strains of bacteria were tested [29].
The chemical investigation of Diaporthe sp., an endophytic fungus associated with the leaves of Rhizophora stylosa collected in Hainan Province, yielded ten new sesquiterpenoids, including six brasilane-type, diaporols J-O (58-63); one 3,6-cycloprecapnellane compound, diaporol P (64); and three drimane ones, diaporols Q-S (65-67) [31]. Among them, compound 66 showed moderate cytotoxicity against the SW480 cell lines with an IC 50  Eutypellol A (68), the first norsesquiterpenoid of the sequicarene family found in nature, was obtained in the marine sediment-derived fungus Eutypella scoparia FS46, which was collected in the South China Sea at a depth of 292 m [12].
The chemical epigenetic manipulation of Eutypella sp. MCCC 3A00281, a deep-sea-sedimentassociated fungus obtained from the South Atlantic Ocean at a depth of 5610 m, resulted in a significant change in the metabolite profile, including the isolation of an array of 26 eremophilane-type sesquiterpenoids, eutyperemophilanes A-Z (69-94), when treated with SBHA and a histone deacetylase inhibitor (HDI) [32]. Remarkably, most of analogs featured a trans fusion of rings A and B, which is uncommon for the eremophilane family in nature [32]. In terms of biological evaluation, compounds 77 and 78 significantly inhibited LPS-activated NO production in RAW264.7 macrophage cells with IC 50 values of 8.6 and 13 µM, respectively [32].  [33]. Interestingly, compound 100 was shown to be a potent anti-inflammatory and weak antiallergic agent, exerting an anti-inflammatory effect with an IC 50 value of 17 µM, which is more potent than that of the p.c. aminoguanidine (IC 50 = 23 µM), and antiallergic activity with an IC 50  Seven new drimane-type sesquiterpenoids, sporulositols A-D (108-111), 6-hydro-xydiaporol (112), seco-sporulositol (113), and sporuloside (114), were later found in the sea mud-derived fungus Paraconiothyrium sporulosum YK-03 collected from the intertidal zone of Bohai Bay in Liaoning Province [35]. Structurally, compounds 108-111 and 113 represent the first five examples of a unique class of drimanic mannitol derivatives, while compounds 113 and 114 might belong to two new series of natural drimanes possessing an aromatic ring with a rare 4,5-secodrimanic skeleton and an unusual CH 3 -15 rearranged drimanic α-d-glucopyranside, respectively [35]. Compounds 108-114 did not show any detectable cytotoxicity against the two tested cell lines A549 and MCF-7 [35].
A chemical investigation was applied to the marine sponge-derived fungus Penicillium adametzioides AS-53, leading to the isolation of two unreported acorane sesquiterpenes, adametacorenols A (133) and B (134) [41]. As shown by cytotoxicity assay, compound 134 exerted selective activity against the NCI-H446 cell line (IC 50  The mangrove-derived fungus Rhinocladiella similis from the fresh leaves of Acrostichum aureum (Pteridaceae) was the source of ten new sesquiterpenoid derivatives, rhinomilisins A-J (138-147), corresponding to one dimeric sesquiterpenoid, four new heptelidic acid derivatives, and five new cadalene-type derivatives [43]. The cytotoxicities of all these isolated compounds were evaluated against the mouse lymphoma cell line L5178Y; compounds 138 and 144 exhibited moderate activity with IC 50 values of 5.0 and 8.7 µM, respectively [43].

Scopulariopsis sp.
Two new phenolic bisabolane-type sesquiterpenes, 11,12-dihydroxysydonic acid (148) and 1-hydroxyboivinianic acid (149), were later found in solid rice cultures of the marine-derived fungus Scopulariopsis sp., which was obtained from the fresh crushed inner tissues of the Red Sea hard coral Stylophora sp. near the coastline of the Ain El-Sokhna area, Red Sea, Egypt [44].
The extraction of Talaromyces minioluteus (Penicillium minioluteum) PILE 14-5, a marine sponge-derived fungus (collected in Pilae Bay, Phi Phi Island, Thailand), led to the production of four new sesquiterpene lactones (156-159), namely, minioluteumides A-D, which are sesquiterpene lactones conjugated with N-acetyl-l-valine that rarely occur in nature [48]. The result of the cytotoxic activity assay of the HepG2 cancer cell lines revealed that compounds 156 and 159 exerted cytotoxicity, with IC 50 values of 50.6 and 57.0 µM, respectively [48].
Research on secondary metabolites of the strain Trichoderma asperellum cf44-2, which is related to the marine brown alga Sargassum sp. collected from the Zhoushan Islands, led to a new bisabolane sesquiterpene, bisabolan-1,10,11-triol (171), and a novel norbisabolane sesquiterpene, 12-nor-11-aceto-xybisabolen-3,6,7-triol (172) [14]. The evaluation of compounds 171-172 for the inhibition of four marine phytoplankton species and four marine-derived pathogenic bacteria (the same test strain used for compounds 169-170) revealed that they all resulted in growth inhibition of the four phytoplankton species tested and weak antibacterial activities were observed against the four tested bacteria, with inhibitory zone diameters of 6.3-7.5 mm at 20 µg/disk [14].

Trichothecium sp.
A culture of the marine-derived fungus Trichothecium roseum from marine driftwood, collected from the intertidal zone of Lingshan Island, was the source of two cyclonerodiol sesquiterpenes: cyclonerodiol C-D (190-191) [55]. The biological evaluation showed that 191 produces moderate antifungal activity against Valsa mali, with a MIC value of 64 µg/mL; meanwhile, 190 exhibited weak bioactivity against V. mali and rhellozoctonia cerealis [55].
Two new sesquiterpenoids, 2α-hydroxyxylaranol B (198) and 4β-hydroxyxylaranol B (199), were obtained from the fermentation of the endophytic fungus J3 isolated from the leaf of the mangrove plant Ceriops tagal collected from the mangrove reserve of Dong Zhai Gang [57].

Diterpenes
Diterpenes are a significant class of terpenoids with diverse structures and notable bioactivities. The current research (25 articles from 2015 to 2019) reported 75 undescribed diterpenes (200-274,  in total from diverse marine fungi compared to 77 diterpenes (42 in 2011-2014, with 35 before 2010) described in 36 papers up to 2014 [7,8], which indicates that the number of diterpenes isolated from marine fungi continues to increase.
The EtOAc extract of Penicillium sp. YPGA11, a deep-sea-sediment fungus collected in the West Pacific Ocean at a depth of 4500 m, resulted in the isolation of three new cyclopiane diterpenes, conidiogenols C-D (247-248) and conidiogenone L (249) [69]. Structurally, compound 247 is the second example of cyclopianes bearing a hydroxyl group at C-13, while 248 is the third example of conidiogenols (coupled with a distinct α-oriented 1-hydroxy group) [69]. After the bioassay study demonstrated the inhibitory effects against five esophageal HTCLs (EC109, KYSE70, EC9706, KYSE30, and KYSE450), compounds 247 and 249 displayed weak inhibitory effects with inhibition rates less than 36% at an initial concentration of 50 µM [69]. Compound 248 exerted moderate antiproliferative effects, with IC 50 values ranging from 36.80 to 54.7 µM (cisplatin as the p.c.) [69].
A new glycosyl ester identified as xylarinonericin E (250) was later found in the fermentation broth of the fungus Penicillium sp. H1 from the sediments of the Beibu Gulf [70]. Compound 250 showed moderate antifungal activity against Fusarium oxysporum f. sp. cubense, with an MIC value of 32.0 µM [70].

Talaromyces sp.
Characterized as a new diterpenoid bearing a novel tetracyclic fusicoccane framework with an unexpected hydroxyl at C-4, roussoellol C (251) was obtained from the laboratory cultures of Talaromyces purpurogenus PP-414 originating from a mud sample collected on a coastal beach [47]. Additionally, compound 251 exhibited moderate antiproliferative activities against four HTCLs (SW480, HL-60, A549, and MCF-7), with IC 50 values ranging from 6.5 to 25.8 µM [47]. Furthermore, 251 showed significant selectivity toward MCF-7 cells, with an IC 50 value of 6.5 µM but an IC 50 value of 10.9 µM against HL-60, contrary to the expectation that cytotoxic natural products will display greater activities against HL-60 cells due to their high sensitivity in the assay [47]. Thus, the selectivity of 251 against MCF-7 makes it a promising lead compound for further study [47].

Trichoderma sp.
The ongoing studies of Liang, X. R. et al. on the fungus Trichoderma citrinoviride cf-27 associated with the fresh tissue of the marine brown alga Dictyopteris prolifera afforded the first Trichoderma-derived and furan-bearing fusicoccane diterpene trichocitrin (252) [71], and a novel norditerpene, citrinovirin (253), with an unprecedented skeleton [72]. The bioactivity results showed that 252 can exert antibacterial activity against E. coli with an inhibitory diameter of 8.0 mm at 20 µg/disc and possesses anti-microalgal capability against Prorocentrum donghaiense with 54.1% growth inhibition at 80 µg/mL [71]. Compound 253 inhibited the growth of S. aureus with an MIC value of 12.4 µg/mL, displayed toxicity against the marine zooplankton Artemia salina with an LC 50 value of 65.6 µg/mL, and additionally achieved 14.1-37.2% inhibition of three marine phytoplankton species (C. marina, H. akashiwo, and P. donghaiense) at 100 µg/mL but promoted the growth of one marine phytoplankton Scrippsiella trochoidea [72].
The fermentation culture of the endophytic fungus Trichoderma sp. Xy24 from the mangrove plant Xylocarpus granatum, collected in the Sanya district, was the source of two new harziane diterpenoids, (9R, 10R)-dihydro-harzianone (254) and harzianelactone (255) [73]. Among them, 254 was the reductive product of harzianone, and 255 was the Baeyer-Villiger monooxygenase-catalyzed oxidation product of harzianone [73]. In terms of in vitro biological evaluation, compound 254 exerted selective cytotoxicity on the HeLa and MCF-7 cell lines with IC 50 values of 30.1 and 30.7 µM, respectively, whereas 255 was inactive at 10 mM [73].
The cultivation of Trichoderma longibrachiatum A-WH-20-2, an endophyte from the marine red alga Laurencia okamurai, yielded one new harziane lactone with an ester linkage between C-10 and C-11, deoxytrichodermaerin (256), which was possibly an oxidation product of harzianone [74]. Deoxytrichodermaerin (256) and the other two isolates (harzianol A and harzianone) were assayed for their inhibition of four marine phytoplankton species (C. marina, H. akashiwo, K. veneficum, and P. donghaiense) with IC 50 values ranging from 0.53 to 2.7 µg/mL [74]. The toxicity of 256 against the marine zooplankton A. salina was also evaluated (LC 50 = 19 µg/mL) and revealed that the lactone unit in deoxytrichodermaerin may contribute slightly to these activities [74].
The chemical investigation was applied to the soft coral-derived fungus Trichoderma harzianum XS20090075 in the South China Sea, resulting in the production of seven secondary metabolites, including two new harziane diterpene lactones possessing a 6/5/7/5-fused carbocyclic core containing a lactone ring system, harzianelactones A and B (257 and 258), and five new harziane diterpenes, harzianones A-D (259-262) and harziane (263) [75]. Specifically, 257 and 258 represent a unique type of harziane diterpene lactone that originated from harziane diterpenes though Baeyer-Villiger monooxygenase-catalyzed oxidation [75]. In the bioactivity study, the above compounds (except for 262) showed obvious phytotoxicity against the seedling growth of amaranth and lettuce at a concentration of 200 ppm [75]. Among them, compounds 257, 259, 260, and 261 were more effective for the complete inhibition of seed germination against amaranth at 200 µg/mL and were still effective at lower concentrations (50 µg/mL) compared to the p.c. glyphosate [75]. However, none was found to inhibit the root growth of lettuce at 200 ppm [75]. In short, the tested compounds seemed to cause weaker inhibition to lettuce than to amaranth and had stronger toxicity on root growth than hypocotyl [75]. Notably, this was the first report of the phytotoxicity of harziane diterpenes from Trichoderma spp. [75].
The search for antagonistic metabolites from Trichoderma asperellum A-YMD-9-2, an endophytic fungal from the marine red alga Gracilaria verrucose, led to the isolation of one new harziane diterpenoid, 3S-hydroxyharzianone (264), which may be an intermediate in the biosynthesis of harziandione from harzianone [52]. Compound 264 could strongly inhibit a wide spectrum of red tide-related phytoplankton species (C. marina, H. akashiwo, K. veneficum, and P. donghaiense) with IC 50 values ranging from 3.1 to 7.7 µg/mL [52]. A structure-activity relationship analysis revealed that the hydroxy group at C-3 of 264 greatly contributes to its inhibitory ability. During the antibacterial assay, 264 exhibited weak inhibition against five marine-derived pathogenic bacteria (four different strains of Vibrio and a P. citrea), at 40 µg/disc [52].
An investigation of the extracts from the fungus Trichoderma asperellum d1-34 that occurs on marine brown alga afforded one unknown diterpene antipode, (+)-wickerol A (270) [77]. The bioactivity assay showed that compound 270 could inhibit E. coli and S. aureus with the same inhibitory diameters of 8.0 mm at 30 µg/disc, and lethal activity was displayed against A. salina with an LC 50 value of 12.0 µg/mL [77].
A new sordarin derivative, trichosordarin A (271), bearing a unique norditerpene aglycone, was discovered from the extracts of a marine sediment-derived fungal strain collected in the Bohai Sea, Trichoderma harzianum R5 [78]. It was assayed to be toxic to the marine zooplankton A. salina with an LC 50 value of 233 µM, but was weak in inhibiting two marine phytoplankton species (Amphidinium carterae and Phaeocysti globosa), with inhibition rates of 20.6% and 8.1%, respectively, at 100 µg/mL [78].
Many new activities were reported for known ophiobolins. Most of ophiobolins show the significant inhibitory activity of tumor cell proliferation and are considered potential antitumor drugs [86]. MHO7 (6-epi-ophiobolin G) can suppress breast cancer cells by downregulating estrogen receptor alpha (ERα), which acts as a novel estrogen receptor degrader [87]. Further research of MHO7 showed that it is appropriate for oral administration according to the target reproductive organs and influences the metabolic pathways via short-chain fatty acids by regulating the levels of the gut microbiome (Ruminococcaceae and Lachnospiraceae) [88]. Another study was also carried out on the antitumor drug candidate ophiobolin O. The results showed that the compound could exert significant antitumor activity, with little in vivo and in vitro toxicity, by inducing G1 phase arrest in the human breast cancer MCF-7 cells related to the AKT/GSK3β/cyclin D1 signaling pathway [89].

Triterpenes
Triterpenes (excluded from steroids) are very rare in marine fungi, and only 11 new compounds were discovered in recent years (including three new molecules, 304-306, in 2015-2019, as shown in Figure 21) [7,8].

Triterpenes
Triterpenes (excluded from steroids) are very rare in marine fungi, and only 11 new compounds were discovered in recent years (including three new molecules, 304-306, in 2015-2019, as shown in Figure 21) [7,8].
Terretonins, a group of polyoxygenated meroterpenoids mostly possessing the same unique tetracyclic scaffolds and primarily isolated from the Aspergillus genus, are derived from DMOA and farnesyl pyrophosphate (FPP) [104]. Three new terretonins with a reversed orientation at H-14, terretonins H and I (331 and 332) [105], along with terretonin D1 (333) [106] were obtained from the marine sediment-derived fungus Aspergillus ustus KMM 4664 and the marine fungus Aspergillus terreus EN-539 associated with the fresh gut of pacific oyster, respectively. Compounds 331 and 332 displayed weak inhibition of fertilized sea urchin eggs [105], and 333 was weakly anti-inflammatory (NO) in RAW264.7 cells [106]. In an alternative study, two new terretonin derivatives, aperterpenes N (334) and O (335), and two known analogs were reported in the marine algal-derived fungus Aspergillus terreus EN-539, among which the stereostructures of the known terretonins A and G were first established by XRD [107]. Compound 334 exhibited inhibitory effects in influenza neuraminidase (IC 50 = 18.0 nM vs. 3.2 nM for oseltamivir), and terretonin G displayed weak inhibition of Micrococcus luteus and Staphylococcus aureus growth (MIC = 8-32 µg/mL vs. 1.0 µg/mL for chloramphenicol) [107].
An unusual stress metabolite induced by Co, aspergstressin (348), possessing a unique fused polycyclic structure, was isolated from the culture broth of the fungus Aspergillus sp. WU 243, collected from a crab dwelling in heavy metal-rich hydrothermal vents [111].
One new merosesquiterpenoid, craterellin D (359), and a known analog craterellin A were isolated from a soft coral-derived fungus, Lophiostoma sp. ZJ-2008011, from which the absolute configuration of craterellin A was first determined [115]. In the bioassays of 359, craterellin A, and the acetonide and acetylation products of craterellin A, only craterellin A showed moderate antibacterial activity against Bacillus cereus, Escherichia coli, Staphylococcus aureus, and Micrococcus luteus (MIC = 3.12-6.25 µM), indicating that the OH in the hydroquinone scaffold may decrease antibacterial activity [115].
In the antibiotic screening experiments, a new metabolite-austinone (373) and six known austin derivatives were discovered from the fungi Penicillium sp. Y-5-2 collected from the sediment of a hydrothermal vent (Kueishantao, Taiwan, China) [120].
Citreohybridonol was re-isolated from the sponge-derived fungus Penicillium atrovenetum and found by NMR to exist as a keto-enol equilibrium tautomer in the solvent [129]. Then, the structure was assigned with the Flack parameter 0.06(3) in X-ray crystallography [129]. Citreohybridonol, re-obtained from the sponge-derived fungus Toxicocladosporium sp. SF-5699, was able to inhibit the production of NO and PGE2 and the expression of iNOS, COX-2, and other pro-inflammatory cytokines, including IL-1β and TNF-α, in the LPS-stimulated BV2 cells [130]. Further experiments demonstrated that citreohybridonol displayed anti-neuroinflammatory activity involved in regulating the TLR4/ MyD88-mediated NF-κB and p38/MAPK inflammatory pathways [130].
The combined chemical analysis strategies of HPLC-HRMS-based hierarchical clustering analysis (HCA) and MS/MS molecular networking were used to isolate the metabolites of the marine fungus Penicillium ubiquetum MMS330 from the blue mussel Mytilus edulis (Loire estuary, France) through six different culture media following the OSMAC approach [133]. Two new meroterpenoids, 22-deoxyminiolutelide A (405) and 4-hydroxy-22-deoxyminiolutelide B (406), together with seven known analogs, were selectively overexpressed and obtained from a seawater CYA (Czapek yeast extract agar) medium and were devoid of cytotoxicity at 50 µM (KB and MCF-7 cells) [133]. Additionally, 22-deoxy-10-oxominiolutelide B was found to easily transform into either compound 405 or 22-deoxyminiolutelide B [133]. Thus, the latter two compounds may be artefacts [133].
The analysis of chemically active components of the mangrove endophytic fungus Talaromyces amestolkiae YX1, cultured on a solid wheat substrate medium, resulted in the isolation of four new DMOA-derived meroterpenoids, the amestolkolides A-D (463-466), which bear a unique and fused polycyclic skeleton with a 6/7/6/5/6 system, along with three known ones, purpurogenolide E and chrodrimanins A and B [154]. Amestolkolides B and A exert potent anti-inflammatory activity by suppressing NO production in LPS-induced RAW264.7 cells (IC 50 = 1.6 and 30 µM, respectively, vs. 26.3 µM for indomethacin) [154].

Conflicts of Interest:
The authors declare no conflict of interest.